There have been known honeycomb structural bodies which support catalyst. Such catalyst is capable of purifying exhaust gas emitted from an internal combustion engine mounted on a motor vehicle, etc. In general, a honeycomb structural body is composed of a plurality of through holes or channels to form cells. The through holes are composed of a plurality of partition walls and cells. That is, the partition walls are arranged in a lattice shape. Each cell is surrounded by the partition walls. A honeycomb structural body is disposed in an exhaust gas pipe through which exhaust gas of a high temperature emitted from an internal combustion engine is discharged to the outside of a motor vehicle. When the exhaust gas passes through the honeycomb structural body which supports catalyst therein, the exhaust gas is purified by the catalyst supported on the surfaces of the partition walls of the honeycomb structural body. For example, such exhaust gas contains harmful substances such as particulate matter PM and Nitrogen oxide NOx. The honeycomb structural body removes such harmful substances from the exhaust gas, and the purified exhaust gas is discharged to the outside of the motor vehicle.
Recently, because the vehicle emissions control of reducing motor vehicle emissions, etc. is becoming stricter year by year, there is a strong demand to reduce harmful substances such as cold emission and hot emissions. Cold emissions mean harmful substances which are generated in and discharged from an internal combustion engine immediately after the internal combustion engine starts to work. The hot emissions mean harmful substances which are generated in and discharged from the internal combustion engine during a high load condition of the engine.
In order to achieve the above recent requirement, the conventional techniques have provided various types of honeycomb structural bodies. For example, a conventional honeycomb structural body has thin partition walls in order to reduce an overall weight of the honeycomb structural body, and to increase the temperature of the honeycomb structural body to a temperature which is necessary to activate catalyst supported on and in the partition walls. Another conventional honeycomb structural body has a constant flow speed of exhaust gas passing through the cells formed in the honeycomb structural body.
Still further, from the viewpoint of the recent fuel efficiency control which becomes further strict, there also is a strong recent demand to decrease a pressure loss of the honeycomb structural body while maintaining the performance of purifying exhaust gas.
For example, a first patent document, Japanese patent laid open publication No. JP 2008-18370, discloses a honeycomb structural body and a relationship between an opening ratio of each of a central section and an outer peripheral section, and a ratio to adsorb water, etc. in the honeycomb structural body.
Further, a second patent document, Japanese patent laid open publication No. JP 2006-281134, discloses a honeycomb structural body having a structure in which a cell opening ratio, etc. is increased continuously or step by step from a central section toward an outer peripheral section.
Further, a third patent document, National publication, Kohyo as Japan Unexamined patent publication No. JP 2009-532605, discloses a honeycomb structural body having a structure in which the number of partition walls formed from a central section toward an outer peripheral section along a radial direction is changed.
Further, a fourth patent document, Japanese patent No. JP 2664118, discloses a honeycomb structural body having a structure in which partition walls forming cells are curved in a radial direction, and a radially outer section of the curved section has a relatively smaller pitch of the partition walls than a radially inner section of the curved section.
Further, a fifth patent document, Japanese patent No. JP 2862298, discloses a honeycomb structural body having a structure in which a plate-shaped section is composed of thin metal plates and a wave-shaped (or curved-shaped) section is composed of thin metal plates. The plate-shaped section and the wave-shaped section are stacked and wound plural times so that a pitch of the wave-shaped section is increased from a start section to an end section.
Still further, a sixth patent document, Japanese patent No. JP 4511396, discloses a honeycomb structural body composed of partition walls. The partition walls have a structure protruded from a central section toward an outer peripheral section, and a central section has a cell density which is greater than that of an outer peripheral section of the partition walls. The sixth patent document further discloses a structure of the honeycomb structural body in which the partition walls have a structure protruded from the outer peripheral section toward the central section, and the central section has a cell density which is lower than that of the outer peripheral section of the partition walls.